The invention relates to a method for making materials for use in memory applications and, more particularly, to a method for making field-structured materials for use in memory applications.
Field-structured materials are a recently-discovered class of materials possessing a substructure of ordered aggregates of suspended dielectric or magnetic particles. This substructure self-assembles under the influence of an external field, such as an external electric or magnetic field, and induces a wide range of mechanical, dielectric, magnetic, and optical properties. When a magnetic particle suspension, consisting of multi-domain particles, is exposed to a uniaxial magnetic field, the magnetic dipole moment on the particles will generally increase and align with the applied field. The particles will then migrate under the influence of the dipolar interactions with neighboring particles, to form complex chain-like structures (Martin et al., J. Chem. Phys., 1998, 108, 3765; Martin et al., J. Chem. Phys., 1998, 108, 7887). If a magnetic particle suspension is instead exposed to a biaxial (e.g., rotating) magnetic field, the induced dipole moments create a net attractive interaction in the plane of the field, resulting in the formation of complex sheet-like structures. Similar effects occur when suspensions of dielectric particles are subjected to uniaxial and biaxial electric fields. These materials, called field-structured composite (FSC) materials, can have large anisotropies in properties such as their conductivity, permittivity, dielectric breakdown strength, and optical transmittance.
Some magnetic particle/polymer composites have been made in the presence of a uniaxial magnetic field. There have been several studies of the magnetic properties of such materials. O""Grady et al (O""Grady et al., J. Magn. and Magn. Mat., 1985, 49, 106) created two different ferrofluids by the thermolysis of di-cobalt octacarbonyl in toluene, controlling the particle size by appropriate surfactant selection. This resulted in a superparamagnetic particle sample, consisting of 5.0 nm particles, and a ferromagnetic sample consisting of 12.0 nm particles, and these particles apparently consisted of essentially single crystalline domains, so that texture could be introduced into the samples by particle alignment. In the superparamagnetic sample, a significant increase in the susceptibility was found when the samples were field-cooled, which oriented the particles in the frozen solvent, leading to significant texture, since each particle consists of essentially a single crystalline domain. In the ferromagnetic particle sample, a significant increase in the remanence was observed in a field-cooled sample, again due to particle rotation along an easy axis creating significant texture. Brugel et al. (Brugel et al., J. Appl. Phys., 1998, 63, 4249) made platelets by ball-milling a thin film of Metglas 2605SC. The platelets were oriented in a magnetic field of 0.4 T, due to the relatively small demagnetizing field in the plane of the platelets, and the polymer resin was then cured. Shifts in the magnetization curves of these materials were found which they attributed to particle alignment, though it is possible that the observed shifts were partly due to the strong local fields produced by particle chains. Jin et al. (Jin et al., IEEE Trans. On Magnetics, 1992, 28, 2211) have investigated uniaxial FSCs of 20 and 75 nm Ni particles coated with a thin layer of Ag.
Hong et al. (U.S. Pat. No. 5,954,991, issued on Sep. 21, 1999) teaches the formation of ordered structures in a thin film of a homogeneous magnetic fluid by exposing the thin film to an external magnetic field. The magnetic fluid consists essentially of Fe2O3 particles coated with a surfactant and dispersed in a non-emulsion liquid, either simple or cyclic hydrocarbons.
A number of researchers have investigated the magnetostriction of uniaxial FSCs, an effect that is at least partly dependent on the magnetic susceptibility anisotropies. Shiga et al. (Shiga et al., J. Appl. Polym. Sci., 1995, 58, 787) created uniaxial FSCs from iron particles in a silicone elastomer and found a large magnetostriction effect along the direction of the structuring field, reported as an increase in the composite shear modulus in a field aligned along the shear gradient, which is also the direction of particle chaining. Anjanappa et al. (Anjanappa et al., Smart Mater. Struc., 1997, 6, 393) and Duenas et al. (Duenas, et al., Mat. Res. Soc. Symp. Proc., 1997, 459, 527) have reported magnetostriction measurements of composites of highly magnetostrictive, field aligned Terfenol-D particles in a polymeric host, and Duenas et al. report chain formation. An enhancement of a factor of roughly two was seen in the magnetostriction in the oriented particle samples. But the focus of these magnetostriction measurements was not on the magnetization properties of the composites, though the two phenomena are related.
Studies of the magnetic properties of sheet-like particle aggregates, such as those that form in rotating fields, are limited. Fabre et al. (Fabre et al., J. Magn. and Magn. Mat., 1990, 85, 77) created sheets of superparamagnetic maghemite particles by swelling a lamellar micelle solution of the surfactant/cosurfactant system sodium dodecyl sulfate/pentanol with nanosize maghemite in cyclohexane to form a lamellar microemulsion. Due to the strong dependence of demagnetizing factors on lamellar orientation, these fluid phases would orient in modest fields (100 Oe), so that the magnetic field is parallel to the lamallae. Measurements of the magnetization of these phases was not reported.